A) Field of the Invention
The present invention relates generally to fiber optics lighting systems and, specifically, to a fiber optics illumination device employing a plurality of optical fibers, said fibers arranged in a spaced and ordered geometrical organization, and a single optical element, for example, a lens, for receiving light from a light source, and distributing said light as a secondary source to said plurality of optical fibers in said organization.
B) Description of the Prior Art
It is well known in the field of optics that optical fibers are capable of effectively and efficiently conducting light from a common source along unequal paths to various locations remote from the light source without encountering substantial transmission losses. Because of this characteristic, there is increasing interest in the application of fiber optics to uses where space is limited. One such use is the overall illumination needs of the interior of vehicles where space is scarce or difficult-to-reach due to aerodynamic and styling considerations.
The prior art discloses various concepts that basically employ a plurality of, for example, 36 or more optical fibers formed into a bundle that is typically 5 to 10 millimeters in diameter. The prior art optical system (FIG. 1) consists of a light source 1, a heat rejection filter 3 and a condenser lens 5. The source 1 is imaged on to the entry face 7a of the fiber bundle 7 by condenser lens 6. The fibers of bundle 7 when viewed from its proximal end 7a is seen to be composed of a plurality of fibers (FIG. 1a). The fibers of bundle 7 may be made of glass or polymer fibers. The individual glass fibers consist of a core 11 surrounded by a cladding 12, the clad fibers arranged in a circular cross-section and constrained within sheath 13. Polymer fiber, on the other hand, may or may not have a cladding depending on the specific application and on the diameter of the polymer core. The core of a polymer fiber may be PMMA and, if clad, a fluoropolymer resin, commonly known by the DuPont trademark Teflon, is used as the cladding material. For small diameter fiber, the polymer fiber's exterior is vacuum coated with a thin film of Teflon.
Said fiber bundle 7 subsequent to entry face 7a is splayed into a plurality of individual fibers, forming the main optical harness, wherein their respective distal ends are separated from each other, arranged and spatially ordered into, for example, a square configuration (FIG. 1b). Ferrules 9 are attached to the distal end of each fiber facilitating a means by which individual fibers of at least one additional fiber optics subharness assembly may be attached. The said subharness, in a conjoined relationship with the main harness, comprises a fiber optics harness assembly for conveying light to courtesy lights, indicator status lights and other lit devices within the vehicle.
Referring to FIG. 2, the fiber optics harness assembly 22 may originate at the rear of the vehicle 20 in, for example, its trunk space. Said fiber optics harness 22 is typically split into two branches 23 and 25, each branch transporting light along opposite sides of the vehicle to the various lit devices 27. The method, by which the individual fibers of fiber bundle 7 are separated, arranged and spatially ordered is a technical problem unique to the application.
There is known a distribution device in which the individual fibers of a fiber bundle are mechanically separated, arranged and ordered. The said device, illustrated in FIG. 3, is a fiber optics bundle connector 30 consisting of five major parts, to wit, a cinch ring 35, a hollow cone 37, a spacer 39, a terminal block 31 and a terminal position assurance device (TPA) 33. Cinch ring 35 is used to tightly pack the individual fibers together into a bundle 7 (FIG. 1a). The hollow cone 37 and its spacer 39, inserted therein, are used to compartmentalize and organize the individual optical fibers into an ordered spatial pattern conducive to their eventual routing destinations within the vehicle. The terminal block 31 provides for path-length equalization of the individual fibers throughout the assembly so that all of the fibers may be terminated individually in the same manner. The TPA 33 ensures that the ferrules of the terminated optical fibers are properly located, thus forming the desired fiber spatial organizaton.
Accordingly, the basic purpose of the fiber optics bundle connector 30 is to transform the fiber bundle 7 into an array of spaced and ordered individual fibers forming a two-dimensional pattern 7b to which one or more fiber optics subharnesses may be attached. More importantly, said arrangement positions and orients said individual fibers of the main harness such that the direction of light propagation emerging therefrom are mutually co-parallel and perpendicular to the distal face of the TPA. In said spaced arrangement and orientation, the individual fibers may be conveniently and efficiently culled and coupled to fibers of one or more fiber optics sub-harnesses, extending from the TPA in route to the various lit devices within the interior lighting system.
While this general approach may fulfill overall illumination requirements, there are major shortcomings to this approach. First, there is attendant to this method significant loss in illuminance due to the number of coupling means required. Second, there is accompanying this method a significant variation in both the luminance and luminous intensity incident on the proximal face of the individual fibers of the main optics harness. The prior art method leads to the non-uniform filling of illumination to the proximal faces of the individual fibers of the sub-harness and, hence, at their exit faces which terminate at the various lit devices. Third, the method requires a relatively large number of parts, requiring assembly, as well as the labor necessary to cull individual fibers into various apertures--a procedure which is not considered to be cost effective.
The luminous efficiencies of the prior art approach will now be discussed. Referring to FIG. 1, the factors affecting the level of illumination received at the distal end of an individual fiber are:
1. Total light accepted by the condenser from the lamp. PA1 2. Reflection losses at air-glass surfaces. PA1 3. Losses at the coupling of light source to fiber bundle PA1 4. Internal transmittance of the fiber. PA1 5. Angle (.alpha.') of the final Illuminating cone. PA1 6. Losses associated with the in-tandem connection of fibers.
If the light source itself, such as a filament-based lamp, presents an area S, the total light accepted by a condenser of aperture .alpha. is given by the formula EQU F=.pi.BS sin.sup.2 .alpha.
The quantity B is a measure of the luminosity of the source, S is the source area and .alpha. is the semi-angle of the angular light distribution of the condenser. The source S is imaged by the condenser on the entry face of the fiber bundle, where the aperture angle is .alpha.'. The linear magnification between this image and the source is given by EQU M=(sin .alpha.)/(sin .alpha.')
so that the magnification for areas is EQU S'/(S)=M.sup.2 =[(Sin .alpha.)/(sin .alpha.')].sup.2
The maximum value of F' exists when the source image just fills the diameter of the entry face of the fiber bundle and sin .alpha.' has a maximum value corresponding to the numerical aperture accepted by the individual fibers of the fiber bundle. Provided the image of the light source completely fills the entry face, and the cones of light forming this image have a sin .alpha.' equal to the numerical aperture (NA) of the fibers, no increase in the amount of light from the source is possible.
When there is loss due to surface reflections, R, the fraction of light transmitted by a single surface is (1-R), and the transmittance of N surfaces is EQU T=(1-R).sup.N
Now, the fraction of light transmitted by the fiber bundle will be considered. A typical arrangement of fibers in a bundle is illustrated in FIG. 1a. Each fiber is composed of a core 11, denoted as the shaded area, surrounded by cladding 12; the interface therebetween providing the necessary total internal reflection action of an optical fiber. The fibers are enclosed within a sheath 13. The ratio of the total area occupied by the fiber cores (shaded areas of FIG. 1a) to the whole area of the bundle defines the core packing fraction (G). The value of this packing fraction is customarily about 0.70, so that only 70 percent of the incident face area of the bundle is active. In addition to this factor in the coupling means, there are reflection losses at the two air-glass surfaces at the ends of the fiber cores. The transmittance of the fiber bundle is seen to be
T=G(1-R).sup.2
Assuming no losses due to imperfect total internal reflection and absorption of the individual fibers.
Imperfect total internal reflection and absorption are accounted for in the measurement of internal transmittance of a fiber of specified length. If T.sub.0 is the internal transmittance of a fiber of length L', the overall transmittance of a fiber of length L is EQU T=G(1-R).sup.2 T.sub.0.sup.(L/L')
Substituting typical values for the factors affecting the level of illumination, where G=0.70, R=0.06 and T.sub.0 =0.80 in white light, it is seen that T=0.40.
There are additional losses associated with the in-tandem connection of the individual fibers 7 and 8 between two fiber optics subharnesses (See FIG. 4). An end-fitting adapter means is used to directly couple and secure two fibers at their respective terminal points. The exit face 7b of each individual fiber on the light source side of the harness assembly must be placed in close proximity to the incident face 8a of each individual fiber on the lit device side of said harness assembly, otherwise light will be lost. For large separations, as illustrated in FIG. 4, some light, shown by the shaded regions between rays L.sub.4 and L.sub.3 and between L.sub.6 and L.sub.7, is lost even from the light cones emerging from the central portion of the fiber.
Light losses are even more significant for light cones emerging from the peripheral regions of the fiber, wherein more than half the light, as shown as the shaded region between L.sub.2 and L.sub.1, fall outside the entry face of the receiving fiber. In practice, this source of error will always be present because the end-fitting adapter does not insure intimate contact between abutting ends of two fibers. The light attenuation factor S, due to fiber separation is estimated, for the typical end-fitting means, to be 90 percent (10 percent light loss). Thus, the overall transmission of two in-tandem connected fibers of length L and L.sub.1, respectively, is given by the expression EQU T.sub.L+L =GS(1-R).sup.4 T.sub.0.sup.(L+L.sub.1.sup.)
Again, substituting typical values for the various factors, it is seen that T.sup.(L+L.sub.1.sup.) is 0.31.
Another source of loss of luminous intensity arising in case of directly coupled fibers is the mismatched radial position between individual fibers. If the exit aperture of each emitting fiber of an in-tandem arrangement is exactly opposite a corresponding receiving fiber, there will be no light lost apart from that hereinbefore described. In practice, some degree of radial mismatch of apertures is unavoidable, thus necessitating the use of the terminal position assurance device 33, shown in FIG. 3.
The non-uniformity of illumination inherent in this approach will now be discussed. It should be noted that, in theory, the luminous flux falling on the entry face of the fiber bundle would be non-uniform. This distribution can be expressed as the quadratic function EQU I(r)=-a r.sup.2 +b
where a and b are constants and r is the radial distance in the entry face of the fiber bundle from its axial center. FIG. 5 is an illustration of the two-dimensional planar light distribution characteristic at the entry face of the fiber bundle. This distribution is not optimal for the following reasons.
The individual fibers receive only the levels of luminosity corresponding to their radial position in the bundle cross-section. The fibers aligned with, and in proximity to the optical axis, receive the maximum flux, whereas the extra-axially positioned fibers progressively receive less light as the bundle-enclosing diameter is approached. The implications are significant. When an individual fiber of the fiber optics bundle is splayed and separated, the irradiance at its distal ends depends on the position (origin) of its proximal end within the bundle and, more specifically, on its proximity to the optical axis, or center, of the fiber bundle. Accordingly, the individual fibers of the main fiber optics harness will emit different levels of luminance. It will, therefore, be seen that the illuminance of the various lit devices within the fiber optics network will vary from one to the other.
The angular light distribution characteristic, in theory, will also be non-uniform. This distribution can also be expressed as a quadratic expression
I(sin .alpha.')=-a(sin .alpha.').sup.2 +b
where a and b are constants and .alpha.' is the semi-angle of the condenser cone of light. FIG. 5a is a two-dimensional representation of the angular light distribution characteristic at the entry face of the fiber bundle. Loss of luminous intensity occurs when .alpha.' exceeds the opening aperture of the individual fibers and when the angular light distribution causes the entrance aperture of the fiber bundle to be either under filled or over filled.
FIG. 6a illustrates that light rays originating from the aperture of light source 6b are incident on the proximal face of the fiber bundle 7a at an oblique angle. Individual fibers, particularly those fibers located near the periphery of the bundle may not accept the entire available incident light because the included incident angle exceeds the numerical aperture of these fibers (shaded areas). Consequently, the extra-axial fibers will transmit even less light to their distal end faces than hereinbefore described. The obliquity factor also affects the angular light distribution of the transmitted light at the distal end of an extra-axial fiber. Light incident on the proximal end of an extra-axial fiber 7a will emerge from its distal end 7b as an annular ring, as shown in FIG. 6b. The semiangle of the exiting cone is seen to contain a hollow center cone. The consequence of this hollow cone of light emerging from the distal ends of extra-axial fibers of the main harness is that, when coupled with the corresponding fibers of the sub-harness, the later fibers will transmit less light, further contributing to non-uniform levels of illumination at respective distal end lit devices.
Finally, light sources, such as tungsten halogen lamps, have complex shapes (See FIG. 7) which emit irrational spatial and angular light distribution characteristics. Such a light source, in combinaton with either a dioptric or catoptic condenser, will produce an image laden with artifacts and structure; further exacerbating the non-uniformity in spatial light distribution at the proximal face of the main harness fiber bundle.
Thus, there is a need for a fiber optics illuminating device that will satisfy the overall illumination requirements of vehicle interior lighting systems while avoiding the aforementioned serious shortcomings.